In recent years, liquid crystal display devices and organic electroluminescent (EL) display devices are widely used as display devices of a smartphone, personal assistant device (PAD), tablet computer, car navigation system, and the like. In general, a liquid crystal display device comprises a liquid crystal panel, and a lighting device (backlight device) overlaid on the rear surface of the liquid crystal panel. The backlight device includes a reflective layer, a light guide plate, an optical sheet, a light source such as an LED, a rectangular frame and the like.
Regarding the liquid crystal display device, a display device provided with a force sensing function of sensing a pressing force applied on the display surface to thereby carry out an arbitrary operation has been proposed. Such a display device adopts an input detection mode of an electrostatic capacitance system of detecting a pressing force, in which a change in a distance between two electrodes provided in the liquid crystal display device is converted into a change in electrostatic capacitance. It is preferable for such a liquid crystal display device that the distance between the two electrodes linearly changes with respect to the pressing force in a thickness direction (an external force in a thickness direction). Further, one electrode is moved or displaced toward the other electrode, the inter-electrode distance changes. Therefore, in order to detect a large pressing force, it is necessary to secure sufficient amount of displacement of the electrode (the amount of stroke).
In order to secure the amount of displacement, an air layer is provided between the electrodes. Furthermore, in order to secure more amount of displacement, an elastic member such as cushion material may be disposed between the electrodes.
In the liquid crystal display comprising a force sensing function utilizing the difference in capacitance between two electrodes as mentioned above, if a relatively thin glass substrate, a cover panel or the like is adopted to slim down the body, the amount of deformation of the glass substrate and the electrodes becomes large according to the inputted pressing force. Under these circumstances, in order to be able to detect a larger pressing force, it is necessary to increase the thickness of the air layer. However, in this case, the thickness of the entire liquid crystal display device is increased, which contradicts the slimming down of the body.
When a cushion material is provided to secure the amount of deformation of the glass substrate and the electrodes, a point of inflection is created in detection of a pressing force between a region deformed within the air layer and a region deformed by the elastic deformation of the cushion material. Further, the point of inflection is varied depending on the place of the display device, more specifically, the place where a pressing force is applied. As a result, it becomes difficult to uniformly detect the pressing force over the entire surface of the display device.